1. Field of the Invention
The invention relates to a process for regenerating noble metal-containing zeolite catalysts which have been deactivated by coke buildup or poisoning. In particular, it relates to a pretreatment process for regenerated catalysts at temperatures from 100.degree. to 250.degree. C. (212.degree.-482.degree. F.). Regenerated catalysts which may be pretreated by the process of the present invention include those that have become deactivated during hydrocarbon hydroprocesses, such as the catalytic dewaxing of hydrocarbon feedstocks.
2. Discussion of the Prior Art
Catalytic dewaxing of hydrocarbon feedstocks, such as distillate fuel oils and gas oils, by isomerization over a Zeolite Beta catalyst is known in the art. U.S. Pat. No. 4,501,926 to LaPierre et al discloses such a process, and is incorporated herein by reference. However, this process requires regeneration to reactivate the isomerization catalyst after being deactivated by coke buildup or poisoning by materials, such as nitrogen, or heavy metals, such as vanadium. Detailed background on catalysis, catalyst poisons and catalyst regeneration and rejuvenation is provided by "Catalyst Deactivation and Regeneration", Chemical Engineering, Vol. 91, No. 23, Nov. 12, 1984. Rejuvenation is generally a reactivation process employing a halogen compound to redisperse agglomerated metal on a catalyst, whereas regeneration is generally a reactivation process not employing a halogen compound.
Processes which utilize chlorine and oxygen in catalyst reactivation are well known. For example, U.S. Pat. No. 3,986,982 to Crowson et al treats deactivated platinum group metal-loaded zeolites by contacting them with a stream of inert gas containing from 0.5 to 20 vol % of free oxygen, and from 5 to 500 ppm volume of chlorine as molecular chlorine, HCl, or inorganic chlorine-containing material. The resulting catalyst is purged to remove residual oxygen and chlorine and then reduced in a stream of hydrogen at 200.degree.-600.degree. C. (392.degree.-1112.degree. F.).
British Pat. No. 1,148,545 discloses a process that is effective for decoking a dual function catalyst, comprising heating catalyst from oxidative burnoff at a temperature of at least 427.degree. C. (800.degree. F.), cooling the catalyst to below 316.degree. C. (600.degree. F.) and partially rehydrating the catalyst, and contacting the partially rehydrated catalyst with hydrogen at a temperature of at least 454.degree. C. (850.degree. F.). However, this involves the hydration and repeated heating and cooling steps, which may cause expansion and contraction of catalysts, and resulting catalyst attrition.
U.S. Pat. No. 3,986,982 to Haag et al discloses catalyst regeneration by contacting the zeolite with hydrogen. The catalyst is contacted with oxygen, pre-coked under controlled conditions, and then contacted with molecular hydrogen under controlled conditions. This process has the drawback of requiring a pre-coking step.
Catalyst regeneration employing oxidation or reduction may be conducted either in situ within a reactor or off-site outside a reactor. Off-site regeneration may comprise contacting a thin layer of catalyst on a moving belt with the oxidizing or reducing gas. There are some benefits to off-site reduction, because it allows high temperature throughout of oxygen without danger of temperature runaway. Also, impurities are removed from the catalyst layer without having to contact other catalysts downstream in the same layer, as in the case for in situ regeneration. Halogen treatment requires certain precautions owing to the corrosive nature of the halogen used. In addition, certain halogen materials employed in these processes add significantly to the cost of catalyst reactivation. In order to avoid the drawbacks associated with halogen use, it would be advantageous to reactivate catalysts in the absence of halogens. However, when deactivating coke present on a catalyst material by exposure to an oxidizing atmosphere of oxygen and an inert gas, such as nitrogen, substantially all of the noble metal on the catalyst may be catalytically inactive.